In 2000 NIOSH conducted a Health Hazard Evaluation (HHE) at a microwave popcorn processing plant after an unusually high percentage of young employees developed obliterative bronchiolitis (OB), a serious lung obstruction disorder. Several of the affected employees subsequently required lung transplant. NIOSH conducted animal studies with the vapor of the flavoring mixture (Hubbs et al., 2002) and observed severe pulmonary effects similar to the OB observed in workers. Because the popcorn process generated vapors containing 0.03-20% diacetyl, NIOSH believes that the OB was caused, at least in part, by diacetyl. [unreadable] [unreadable] It was assumed initially that the OB observed in workers was caused by acute exposure to a high concentration of diacetyl. However, a recent epidemiological study (Kreiss et al., 2002) revealed that acute exposures could not be documented and the pulmonary effects likely resulted from repeated exposures to low concentrations. The possibility that inhalation of low, non-irritating concentrations of diacetyl could cause sudden, severe respiratory effects, increases the level of concern for workers. Because of the renewed concerns for diacetyl exposure, inhalation toxicity studies of diacetyl in animals are in progress.